Engineering, Universiti Teknologi Malaysia, UTM Skudai, Johor Bahru, Malaysia

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1 Co-Generation of C 2+ Hydrocarbons and Synthesis Gas from Methane and Carbon Dioxide using Dielectric-Barrier Discharge Plasma Reactor without Catalyst at Low Temperature Istadi 1,2 and Nor Aishah Saidina Amin 1,* 1 Chemical Reaction Engineering Group (CREG), Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, UTM Skudai, Johor Bahru, Malaysia ABSTRACT 2 Permanent Address: Chemical Reaction Engineering & Catalysis (CREC) Group, Dept. of Chemical Engineering, Diponegoro University, Semarang, Indonesia istadi@tekim.ft.undip.ac.id Co-generation of C 2+ hydrocarbons and synthesis gas from methane and carbon dioxide has been an important issue in tackling the global warming effects from the two greenhouse gases. Dielectric-barrier discharge (DBD) plasma technology has been proposed to improve the process involving conversion and utilization of both methane and carbon dioxide. In this paper, a study on the effects of CH 4 /CO 2 feed ratio, total feed flow rate, and discharge voltage on performance of DBD plasma reactor without catalyst at low temperature were addressed. The three factors in the DBD reactor showed significant effects on the reactor performances, i.e. methane conversion, synthesis gas and C 2+ hydrocarbons selectivities. With the DBD plasma reactor without catalyst, the C 2+ hydrocarbons (ethane, ethylene, acetylene, and propane) and synthesis gas (hydrogen and carbon monoxide), were produced from methane and carbon dioxide at low temperature with promising performances. Co-feeding carbon dioxide to the methane feed stream reduced coking and enhanced methane conversion. Methane and carbon dioxide conversions were influenced significantly by the discharge voltage. High discharge voltage, low CH 4 /CO 2 feed ratio, and low total feed flow rate were suitable for the co-generation over DBD plasma reactor with potential performances. Keywords: dielectric-barrier discharge, plasma discharge reactor, CH 4 -CO 2 conversion, synthesis gas 1. INTRODUCTION The studies on conversion and utilization of methane and carbon dioxide are widely researched in the field of C1 chemistry. Several technologies have been proposed to improve the efficiency and utilization of methane and carbon dioxide. Mitigation of CO 2, one of the most important greenhouse gases, is the crucial agenda in global warming issues. Meanwhile, the direct conversion of methane to C 2+ hydrocarbons and synthesis gas has a large implication towards the utilization of natural gas in the gas-based petrochemical and liquid fuel. The CO 2 and CH 4 contents in Natuna s, Arun s and Terengganu s natural gases with CH 4 /CO 2 ratio 28/71, 75/15 and 80/8, respectively should be strategically utilized for the production of synthesis gas, higher hydrocarbons, liquid fuels and other important chemicals [1,2].The potentials of non-conventional DBD plasma reactor for converting the two greenhouse gases, CH 4 and CO 2, to higher hydrocarbons and synthesis gas at low temperature and ambient pressure have also been reported [3]. A comprehensive review on recent development of plasma reactor technology for the co-generation of synthesis gas and C 2+ hydrocarbons from methane and carbon dioxide has been reported to address the features, drawbacks, challenges, and feasibility of this technology [4]. The oxidative coupling of methane (OCM) is a promising and novel route for the conversion of natural gas to C 2 hydrocarbons in the presence of a basic catalyst within the temperature range of 923 to 1127 K. Recently, carbon dioxide was used to replace oxygen as an oxidant in oxidative coupling of methane by carbon dioxide (CO 2 OCM) to produce active oxygen species which promotes formation of * Corresponding Author. noraishah@fkkksa.utm.my. Tel , Fax

2 CH * 3 radicals from methane. Unlike O 2, CO 2 does not induce gas phase radical reactions. It is thus expected that the development of active catalysts achieves high selectivity to C 2 hydrocarbons. From the thermodynamic studies, production of C 2 H 6 and C 2 H 4 from CH 4 and CO 2 showed low performances due to thermodynamic limitation [5,6]. Pertaining to CO 2 OCM reaction, various binary metal oxides have been proposed, i.e. CaO-CeO 2 [5], SrO-MnO 2 [7], and MnO 2 -SrCO 3 [8]. Almost all the catalysts achieved low methane conversion (up to 7%), high enough C 2 hydrocarbons selectivity (up to 85%), and low C 2 hydrocarbons yield (up to 4.5%). The CH 4 and CO 2 reactions over these catalysts only produce C 2 H 6, C 2 H 4 and CO without any other products such as H 2, C 2 H 2, or C 3 H 8. In addition with respect to ternary metal oxide system, CaO-MnO/CeO 2 catalyst attained C 2 selectivity of 76.6% and C 2 yield of 3.7% using an optimization strategy [9]. The low performance of the CO 2 OCM reaction is related to o high G T at all temperature range which showed that the reactions are no favoured thermodynamically. The major intricacy for direct methane conversion to higher hydrocarbons and oxygenates involve the intensive energy consumption requirement and also activating the stable C-H bonds in the methane molecule using conventional catalysis. Due to the thermodynamic constraints in obtaining high yield for the CO 2 OCM, further improvements are required including the exploitation of some non-conventional technologies such as plasma reactor. In this paper, a study on the effects of CH 4 /CO 2 feed ratio, total feed flow rate, and discharge voltage on performance of DBD plasma reactor without catalyst at low temperature are addressed. The effect of the process variables on the CH 4 conversion, selectivity and yield of C 2+ hydrocarbons, and H 2 /CO ratio are considered. 2. EXPERIMENTAL METHOD 2.1. Experimental Rig Set-up of DBD Plasma Reactor The schematic diagram of experimental rig set-up of DBD plasma reactor is depicted in Figure 1, while Figure 2 presents the schematic diagram of high voltage generator circuit. A high voltage pulse AC generator supplies a voltage in the ranges 0-17 kv with a pulsed waveform at a frequency of up to 10 khz (Atama Tech Sdn Bhd). The voltage measurement was conducted using an oscilloscope (ISO-TECH ISR 622) equipped with a high voltage probe (Atama Tech Sdn Bhd). All experimental works were conducted at fixed pulse frequency (2 khz). 2 Figure 1. Schematic diagram of experimental rig set-up of DBD plasma reactor: (1). Ball valve; (2). Volumetric flow controller; (3). Check valve; (4,5). Four and three way valves; (6). DBD plasma reactor; (7). High voltage probe; (8). DC power supply; (9). High voltage generator; (10). Oscilloscope; (11). Condensor; (12). Online GC; (13). Computer for GC; (P). Pressure gage The high voltage pulse AC generator circuit as depicted in Figure 2 can be divided to two main sections: the oscillator and the power drive. The oscillator is built around a CMOS 4093 (4-nand gates) and is configured as a pulse generator (duty cycle controlled). Either on or off time may be set by using

3 the 150 kω potentiometer (high and low potentiometers). The main working frequency band can be set by proper selection of the capacitor 10 nf. The on/off time control gives a very good regulation to fine-tune the output for maximum output voltage. In fact, once the frequency was increased, the voltage was decreased, and vice versa. The power drive is built around a power switching transistor (2N3055) which is driven by a 2N2222 transistor. The power transistor receives the command signal from the oscillator that opens and closes many times per second the primary coil of the transformer, thus inducing a high voltage in the transformer secondary coil. The output voltage is dependent on the transformer type, power transistor, and power supply Design of Experiment (DOE) Figure 2. Schematic diagram of high voltage generator circuit A Central Composite Rotatable Design (CCRD) for three independent variables was employed to design the experiments [9]. The design is intended to reduce the number of experiments and to arrange the experiments with various combinations of independent variables. The ranges and levels used in the experiments are given in Table 1. The experimental design matrix resulted by the CCRD are revealed in Table 2 involves 19 sets of coded conditions expressed in natural values. The design consists of 8 cube points of two-level full factorial design, 6 star points and 5 centre points. In this paper, effect of three variables on the performances of plasma CH 4 -CO 2 conversion is presented in three dimensional surface profiles. The surface profile was prepared based on the experimental results with respect to the CCRD design by utilizing Statistica software (Statsoft, Inc.). The profiles were produced in the software by implementing an empirical quadratic polynomial approach to the experimental data with reasonable determination coefficient. Table 1. Ranges and levels of factors or independent variables used in experiments Range and levels Factors -α (-1.68) α (+1.68) CH 4 /CO 2 Ratio, [-] Discharge voltage, kv Total feed flow rate, cm 3 /min Note: -1 (low level value); +1 (high level value); 0 (center point); +α and -α (star points)

4 3. RESULTS AND DISCUSSION Principles of DBD Plasma Reactor The principle is based on high energy electrons. Non-thermal plasma can be defined as gas consisting of electrons, highly excited atoms and molecules, ions, radicals, photons and neutral particles in which the electrons have a much higher energy than the neutral gas particles. Non-thermal plasma is also called non-equilibrium plasma due to the significant difference of temperature or kinetic energy between the electrons and the neutral particles [3]. The gas temperature can be within the range of room temperature, while the electrons can reach temperatures of K in a dielectric-barrier discharge. The non-thermal plasma can be generated and maintained by electrical discharge [3] which is a direct way to produce non-thermal plasma by applying a high voltage to a gas space and incurring gas breakdowns. The gas breakdowns generate electrons that are accelerated by an electric field forming non-thermal plasma. The electrical discharges can be realized in several ways depending on the types of voltage applied and reactor specification. In the plasma reactor, the energetic electrons collide with molecules in the gas, resulting in excitation, ionization, electron multiplication, and the formation of atoms and metastable compounds [3,10]. When the electric field in the discharge gap is high enough to cause breakdowns in most gases, a large number of microdischarges are observed. The active atoms and metastable compounds subsequently collide with molecules, and reactions may occur. For CO 2 reforming of methane, it is expected that CH 4 and CO 2 participate in the reactions and be converted into higher hydrocarbons and synthesis gases. The possible reactions occurred in the CH 4 and CO 2 conversions over the DBD plasma reactor without catalyst at low temperature can be described as follows [3]: CH 4 + e CH * 3 + H * + e (1) CO 2 + e CO + O * + e (2) CH * 3 + CH * 3 C 2 H 6 (3) C 2 H 6 + e C 2 H * 5 + H * + e (4) C 2 H * 5 + CH * 3 C 3 H 8 (5) H * + H * H 2 (6) C 2 H * 5 + e C 2 H 4 + H * + e (7) C 2 H 4 + e C 2 H * 3 + H * + e (8) C 2 H * 3 + e C 2 H 2 + H * + e (9) C 2 H * 5 + C 2 H * 5 C 4 H 10 (10) C 4 H 10 + e C 4 H 9 + H * + e (11) H 2 + e 2H * + e (12) CH * 3 + e CH * 2 + H * + e (13) CH * 2 + e CH * + H * + e (14) CH * + CH * C 2 H 2 (15) CH * 2 + CH * 2 C 2 H 4 (16) CH * + e C + H * + e (17) In the reactions, e denotes high energy electron, while * express radical species from dissociation reaction Experimental Data of DBD Plasma Reactor without Catalyst at low temperature The experimental results of DBD plasma reactor without catalyst at low temperature are listed in Table 2. The experimental data is presented based on design of experiment (DOE) with three independent variables, CH 4 /CO 2 ratio, discharge voltage, and total feed flow rate. The CCRD design is practical for addressing the effect of each independent variable as well as the interaction among the variables [9]. Gaseous hydrocarbons (C 2 -C 3 ), hydrogen and CO are the main products in the gas phase. The CO is principally produced from the dissociation of carbon dioxide, while the light hydrocarbons and hydrogen are the primary products from methane conversion. Pertaining to the C 2+ hydrocarbons

5 products, the concentration of ethane (C 2 H 6 ) is much higher than that of ethylene (C 2 H 4 ), acetylene (C 2 H 2 ) and propane (C 3 H 8 ) in the gaseous products. The results are significantly different with the results using corona discharges [3,12] where acetylene (C 2 H 2 ) is the primary product. Direct conversions of CH 4 and CO 2 to higher hydrocarbons and synthesis gases is promising over the DBD plasma reactor without catalyst at low temperature. In fact, the conversions using conventional catalytic fixed bed reactor at high temperature (about 850 o C) only produced C 2 H 6, C 2 H 4 and CO with very low CH 4 conversion, namely CO 2 OCM [9]. In other catalytic process (carbon dioxide reforming of methane), it only produced H 2 and CO at high temperature (about 850 o C) over suitable catalyst with considerable methane conversion. Table 2. Experimental data of DBD plasma reactor without catalyst at low temperature CH 4 /CO 2 ratio Process variables Discharge voltage (kv) Total feed flow rate (cm 3 /min) X CH 4 X CO 2 Responses/Dependent variables S C 2+ S H 2 S CO Y C 2+ Y H 2 Y CO 5 H 2 /CO Ratio Note: - X, S, Y denote conversion, selectivity and yield, respectively - C 2+ comprises C 2 H 4, C 2 H 6, C 2 H 2, C 3 H Effect of Operating Conditions on CH 4 Conversion, C 2+ Hydrocarbons Selectivity and Yield over DBD Plasma Reactor CH 4 conversion, C 2+ hydrocarbons selectivity and yield are significantly affected by discharge voltage, CH 4 /CO 2 ratio and total feed flow rate as depicted in Figures 3-5. From Figure 3, the CH 4 conversion enhances with concentration of CO 2 in the feed. As an oxidant, CO 2 is first decomposed to CO and O * by high energy electrons. The hot electrons collide with CO 2 molecules and excite them to higher energy levels to dissociate or initiate the reactions. The dissociation reaction of CO 2 generates some oxygen species. Some excited atomic species such as metastable oxygen species (O( 1 D)) are active species for the generation of methyl radicals from CH 4 via dissociation reaction. The dissociation of CO 2 will generate oxygen active species that assist plasma catalytic methane conversion. With co-feed of CO 2, the reaction of methane species is deeply influenced by the new reactive oxygen species so that the conversion of methane is much higher than CO 2 conversion as exhibited in Table 2. The possible mechanism is that the reactive oxygen removes H atom in methane to generate the hydrocarbons radicals [3]. The excited species will react with the hydrocarbons radicals in the plasma zone. Indeed, the co-feed CH 4 and CO 2 promote the conversion each other. The distribution of hydrocarbon products changes significantly with the CH 4 /CO 2 feed ratio [10]. It is suggested that the

6 CH 4 concentration in the feed is an important factor for the total amount of hydrocarbons produced. Lower amount of CO 2 in the feed favours the production of C 2+ hydrocarbons as revealed in Figures 4 and 5. From Table 2, the larger the CO 2 amount in the feed, the higher the CO selectivity, whereas the selectivity of all higher hydrocarbons decreased. This phenomenon is corroborated with the results of Zhang et al. [10]. The yield of gaseous hydrocarbons (C 2+ ) increases considerably with the CH 4 /CO 2 feed ratio as exhibited in Figure 5. It is possible to control the composition of C 2+ hydrocarbons and hydrogen products by adjusting the CH 4 /CO 2 feed ratio. It is recommended that the range of CH 4 /CO 2 feed ratio 2 3 should be used to get a high selectivity of C 2+ hydrocarbons and hydrogen with high methane conversion. Further increasing CH 4 /CO 2 ratio above 3 favours lower CH 4 conversion and coking formation. In this work, the composition of feed gas is an important factor to influence the product distribution. Obviously, more methane in the feed will produce more hydrocarbons. The drawback of DBD plasma methane conversion with pure CH 4 feed is the formation of carbon deposits on the surface of electrode. The co-feed of CO 2 can inhibit the deposition of carbon and thus sustain the operations of DBD. The coke is mainly formed via CH 4 decomposition, not CO disproportionation. The dissociation energy of CO possessing of 11.1 ev is higher than that of dissociation energy of CH x (x=1-4), which is lower than 5 ev [12]. The CO 2 is more difficult to be dissociated via the collisions of electrons than CH x. 6 (a) (b) Figure 3. Effect of operating conditions on CH 4 conversion: (a) discharge voltage and total feed flow rate vs CH 4 conversion presented at CH 4 /CO 2 ratio of 3; (b) discharge voltage and CH 4 /CO 2 ratio vs CH 4 conversion presented at total feed flow rate of 40 cm 3 /min Total feed flow rate affects the residence time within the discharge zone. Increasing the total feed flow rate reduces the conversion of methane and carbon dioxide quickly as demonstrated in Figure 3(a) [12]. In fact, increasing the total feed flow rate does not affect significantly on the C 2+ hydrocarbons and hydrogen selectivity. Fincke et al. [13] concluded that an appropriate ratio of CH 4 /CO 2 in the feed, a relatively low discharge voltage, and a relatively large total feed flow rate are benefit for higher energy efficiency. Actually, the low total feed flow rate with high residence time leads to the high intimate collision among the gas molecules and high energy electrons. The high intensive collisions favour the methane and carbon dioxide conversions to C 2+ hydrocarbons. The various kinds of electric sources were used in plasma reactor such as AC, DC, microwave, or pulse power supply operated at high voltage and high frequency AC power. Varying the discharge power/voltage affects predominantly on methane conversion and higher selectivity of higher hydrocarbons (C 2 -C 3 ). The conversions of CH 4 and CO 2 increase with discharge voltage as exhibited in Figure 3. At high discharge power the CH 4 conversion becomes higher than that of CO 2 at higher discharge voltage since the dissociation energy of CO 2 (5.5 ev) is higher than that of CH 4 (4.5 ev) [3,14]. More plasma species are generated at higher discharge voltage. In addition to high energy electron, species such as H, OH, O radicals can attack CH 4 molecules to produce more methyl radicals. The selectivity of C 2+ hydrocarbons decreases slightly with increasing the discharge voltage at fixed

7 frequency as revealed in Figure 4 which is corroborated with the result of Liu et al. [15] and Song et al. [16]. This means that the increased discharge power may destroy light hydrocarbons (C 2 -C 3 ). The low discharge power is beneficial to obtain higher selectivity to light hydrocarbons (C 2 -C 3 ), but the CH 4 conversion is low. Eliasson et al. [17] reported that higher discharge power is necessary for generating higher selectivity to higher hydrocarbons (C 5+ ) over DBD reactor with the presence of zeolite catalysts. Higher discharge power is suggested for efficient methane conversion. As the discharge power increases, the bulk gas temperature in the reaction zone increases. 7 Figure 4. Effect of discharge voltage and CH 4 /CO 2 feed ratio on C 2+ selectivity presented at total flow rate of 40 cm 3 /min Figure 5. Effect of discharge voltage and CH 4 /CO 2 feed ratio on C 2+ hydrocarbons yield presented at total feed flow rate of 40 cm 3 /min Based on the effect of operating variables studies, it is recommended that low total feed flow rate, high CH 4 /CO 2 feed ratio and high discharge voltage are suitable for producing C 2+ hydrocarbons and synthesis gas. Table 3 shows the results of DBD plasma reactor at recommended conditions and their comparison with pure CH 4 as a feed. From the table, it is shown that CH 4 /CO 2 ratio = 3 provides more promising results. However, increasing CH 4 /CO 2 ratio to 4 reduces methane conversion considerably

8 and leads to enhanced C 2+ hydrocarbons selectivity and H 2 /CO ratio slightly. It is confirmed that CO 2 as co-feed has an important role in improving CH 4 conversion by contributing oxygen active species from the CO 2. The coking formation was appeared obviously in the surface of electrode at CH 4 /CO 2 ratio of 4. The significant decreasing CH 4 conversion from 21.95% to 15.23% is also reported for pure CH 4 as a feed with higher C 2+ hydrocarbons selectivity. The coking formation is obvious distinctly in the surface of electrode. In this case, CO 2 also helps the reduction of coking formation. It can be concluded that too high CH 4 /CO 2 ratio is not suggested in the operation of DBD plasma reactor due to decrement of CH 4 conversion and coking formation reasons. Table 3. Results of DBD plasma reactor without catalyst at potential operating conditions and comparison with that at pure CH 4 as feed. CH 4 /CO 2 Ratio Process variables Discharge Voltage (kv) Total Feed Flow Rate (cm 3 /min) X CH 4 X CO 2 Responses/Dependent variables S C 2+ S H 2 S CO Y C 2+ Y H 2 Y CO 8 H 2 /CO Ratio Pure CH Effect of DBD Plasma Conditions on H 2 /CO Synthesis Gas Ratio Figure 6 presents the effect of discharge voltage and CH 4 /CO 2 feed ratio on H 2 /CO synthesis gas ratio. From the figure, the H 2 /CO product ratio is mainly controlled by the discharge voltage and CH 4 /CO 2 feed ratio. Increasing the discharge voltage leads to an increase in the H 2 /CO ratio slightly and achieves maximum at 13.5 kv. Next increasing the discharge voltage decreases the H 2 /CO ratio slightly due to more CO is produced from CO 2 and CH 4 which is in agreement with the result of Song et al. [16] without catalyst. The H 2 /CO ratio increases considerably with CH 4 concentration in the feed gas. This means that once CH 4 concentration in the feed gas increased more H 2 was produced and less CO in the product. It is possible to control the composition of the synthesis gas product by adjusting the molar ratio of feed gases. Zhang et al. [11] reported that lower feed flow rate and high discharge voltage is beneficial for producing synthesis gases with higher H 2 /CO ratio and for obtaining high selectivity to higher hydrocarbons. Figure 6. Effect of discharge voltage and CH 4 /CO 2 feed ratio on H 2 /CO synthesis gas ratio presented at total feed flow rate of 40 cm 3 /min

9 4. Conclusions 9 A study on the effects of CH 4 /CO 2 feed ratio, total feed flow rate, and discharge voltage on the performance of DBD plasma reactor without catalyst at low temperature were successfully addressed. The three factors in the DBD plasma reactor showed significant effects on the reactor performances, i.e. methane conversion, synthesis gas selectivity, C 2+ hydrocarbons selectivity. A lower feed flow rate was appropriate for producing high selective C 2+ hydrocarbons and higher H 2 /CO ratio of synthesis gases, because of high residence time of the gases within the discharge zone. The range of CH 4 /CO 2 feed ratio being 2 3 is suitable in the DBD reactor to obtain high selective C 2+ hydrocarbons and hydrogen with high methane conversion. High discharge power/voltage is required to obtain high yield C 2+ hydrocarbons. It is concluded that too high CH 4 /CO 2 ratio is not suggested in the operation of DBD plasma reactor due to decrement of CH 4 conversion and coking formation. The application of the DBD plasma reactor is potential for the co-generation of C 2+ hydrocarbons and synthesis gases from methane and carbon dioxide. Future development by introducing suitable catalysts is required in order to improve the C 2+ hydrocarbons selectivity. Acknowledgment The authors would like to express their sincere gratitude to the Ministry of Science, Technology and Innovation, Malaysia for the financial support received under the IRPA Project No EA099. The authors thank Encik Suzaimi and Encik Yap (ATAMA Sdn Bhd) for fabrication of high voltage generator and high voltage probe, respectively. References [1] Suhartanto, T., York, A.P.E., Hanif, A., Al-Megren, H. and Green, M.L.H. (2001). Potential Utilisation of Indonesia s Natuna Natural Gas Field via Methane Dry Reforming to Synthesis Gas. Catalysis Letters. 71: [2] Centi, G., Cavani, F., Trifirò, F. (2001). Selective Oxidation by Heterogeneous Catalysis. New York: Kluwer Academic/Plenum Publishers [3] Liu, C.J., Xu, G.H., and Wang, T. (1999). Non-Thermal Plasma Approaches in CO 2 Utilization. Fuel Processing Technology. 58: [4] Istadi, and Amin, N.A.S. (2005). Co-Generation of Synthesis Gas and C 2+ Hydrocarbons from Methane Carbon Dioxide Reaction in A Hybrid Catalytic-Plasma Dielectric-Barrier Discharge Reactor: A Review. Fuel. Under Review. [5] Wang,Y., Takahashi, Y., and Ohtsuka, Y. (1999). Carbon Dioxide as Oxidant for the Conversion of Methane to Ethane and Ethylene Using Modified CeO 2 Catalyst. Journal of Catalysis. 186: [6] Istadi, and Amin, N.A.S. (2005). Co-generation of C 2 Hydrocarbons and Synthesis Gases from Methane and Carbon Dioxide: A Thermodynamic Analysis. Submitted to Journal of Natural Gas Chemistry. [7] Wang, Y., and Ohtsuka, Y. (2001). Mn-based Binary Oxides as Catalyst for the Conversion of Methane to C 2+ Hydrocarbon with Carbon Dioxide as an Oxidant. Applied Catalysis A: General. 219: [8] Cai, Y., Chou, L., Li, S., Zhang, B. and Zhao, J. (2003). Selective Conversion of Methane to C 2 Hydrocarbons Using Carbon Dioxide over Mn-SrCO 3 Catalysts. Catalysis Letters. 86(4): [9] Istadi and Amin, N.A.S. (2005). A Hybrid Numerical Approach for Multi-Responses Optimization of Process Parameters and Catalyst Compositions in CO 2 OCM Process over CaO-MnO/CeO 2 Catalyst. Chemical Engineering Journal. 106: [10] Zhang, K., Kogelschatz, U. and Eliasson, B. (2001). Conversion of Greenhouse Gases to Synthesis Gas and Higher Hydrocarbons. Energy & Fuels. 15: [11] Zhang, K., Eliasson, B. and Kogelschatz, U. (2002). Direct Conversion of Greenhouse Gases to Synthesis Gas and C 4 Hydrocarbons over Zeolite HY Promoted by a Dielectric-Barrier Discharge. Industrial and Engineering Chemistry Research. 41:

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